This invention relates to cellulose solutions, and to the preparation of cellulose fibers, films, foams and fabricated articles therefrom, and to the direct extraction of cellulose from lignocellulosic materials.
The dissolution of cellulose has been a primary objective of workers in the art since the very dawn of cellulose chemistry. Cellulose solutions form the basis for the commercial manufacture of cellulose fibers, films, foams, and other cellulose molded, shaped or otherwise manufactured articles or products (referred to herein as "fabricated articles").
Because of its great crystallinity, relatively rigid backbone and high melting point (decomposition temperature) cellulose is more difficult to dissolve than related polysaccharides, e.g., starch and dextran. The known solvents for cellulose effect dissolution by first forming a cellulose salt or complex, e.g., a cellulose nitrate, acetate, xanthate, ether or metallo complex. Cellulose solvents include various concentrated mineral acids, some salt solutions at high concentrations and elevated temperatures, various organic amines and ammonium and diamine complexes of heavy metals such as iron, cobalt, nickel, copper, cadmium or zinc, e.g., cuoxam [tetramminocopper dihydroxide [Cu(NH.sub.3).sub.4 ](OH.sub.2)], or cuene [bis(ethylenediamine) copper dihydroxide [NH.sub.2 CH.sub.2 CH.sub.2 NH.sub.2 ].sub.2 Cu(OH).sub.2 ], dibenzyldimethylammonium hydroxide, nitrogen tetroxide in acetonitrile or dimethylformamide, or the like. [See Encyclopedia of Polymer Science & Technology, Kirk-Othmer, 3 (1965), page 166; Encyclopedia of Chemical Technology, Kirk-Othmer, 2nd Ed., 4, (1965), page 601; Polymer Handbook, Brandrup & Immergut, Chapter VI, "Properties of Celluluse Materials", pages 14-16.]; and U.S. Pat. Nos. 1,943,176 and 3,305,499.
The use of conventional techniques such as the viscose and acetate processes for the manufacture of cellulose and cellulose derivative fibers and other products currently faces critical problems due to ever-increasing capital investment, energy and pollution control costs.
The use of various solvent-based processes for dissolving cellulose and manufacturing fibers, films, foams or other fabricated articles therefrom is subject to the further disadvantage that solution is effected in these procedures by reaction to form a more soluble cellulose salt or complex-- materials thus dissolved are mixed products from which pure cellulose must be recovered by expensive purification and recovery operations, or which solutions exhibit decreased thermal stability or increased corrosive characteristics, thus frequently precluding their use for the preparation of cellulose fibers or foams or in similar applications.
For example, U.S. Pat. No. 3,424,702 discloses dissolving cellulose in mixtures of liquid sulfur dioxide with aliphatic secondary or tertiary, or alicyclic, amines. It is believed that these mixtures effect dissolution by forming sulfite half esters with the --OH groups of the cellulose, the amine then forming a salt or strongly hydrogen bonding with the sulfite. Esterification breaks up the crystallinity of the cellulose molecule and permits dissolution.
The compound thus formed is, however, unstable, breaking down (with degradation) slowly at room temperatures and more rapidly at elevated temperatures. Because of this thermal instability cellulose fibers cannot be readily prepared from such solutions.
Accordingly, it is among the objects of the present invention to provide cellulose solutions which may be used in the manufacture of cellulose fibers, films, foams and other fabricated articles without the magnified capital investment, energy and pollution control problems to which existing processes are increasingly subject. A further object of the invention is to provide cellulose solutions without the use of reactive solvents, which solutions may be directly utilized employing known techniques in the production of cellulose products. Other objects and advantages of the invention will become apparent from the following description of preferred embodiments thereof.